This invention relates to a torch for fabricating an optical fiber perform in case of manufacturing a porous glass preform for a communication or optics through a VAD method or an OVD method.
A VAD method or an OVD method which mixes less impurities and OH group is employed as means for fabricating a porous glass preform for an optical fiber, an image fiber, a light guide or a rod lens.
Each of the above-mentioned methods supplies raw gas, combustible gas and combustion supporting gas or these gases and sealing gas to a torch for fabricating a porous glass preform, produces soot-state porous glass preform by flame hydrolysis and/or thermal oxidation, and accumulates the preform in a desired shape such as a rod or tube shape.
A torch used in these methods has a multiwall tube structure of triple or more wall tubes. When the torch is formed, for example, of a quadruple wall tube structure, the passages from the center to the outermost periphery of the torch are used as a raw gas injection passage (first passage: at the center), a sealing gas injection passage (second passage), a combustible gas injection passage (third passage) and a combustion supporting gas injection passage (fourth passage: the outermost periphery).
The raw gas contains SiCl.sub.4 of main raw gas, and GeCl.sub.4, POCl.sub.3, BC1.sub.3, of doping raw materials. The combustible gas contains hydrogen (H.sub.2), methane, propane, butane or a mixture gas of any two or more gases. The combustion supporting gas contains oxygen (O.sub.2), and the sealing gas contains Ar and/or other inert gas.
Principles of accumulating the optical fiber preforms in the VAD and OVD methods are fundamentally the same, but the VAD method accumulates the optical fiber preform on the lower end of a vertical target drawn while rotating, and the OVD method accumulates the optical fiber preform on the outer periphery of a mandrel rotating in a horizontal state.
The porous glass preform thus accumulated and formed through the above methods is dehydrated and transparently vitrified by the following heat treatment to become a transparent preform which contains no air bubble.
In case of the above-mentioned VAD method, the porous glass preform is grown axially by the accumulation of the optical fiber preform. In this case, as the preform is grown, a large own weight is applied to the preform. Thus, when a long and large porous glass preform is produced, the preform tends to be damaged by the weight of itself.
Therefore, it is necessary to improve the strength of the preform to enhance the accumulating density of the optical fiber preform when fabricating the large-size porous glass preform by the VAD method.
In case of the OVD method for accumulating an optical fiber preform on the outer periphery of a mandrel of horizontal state, no damage occurs in the porous glass preform as observed in the VAD method, but as the optical fiber preform is accumulated, the diameter of the preform increases so that the surface area of the preform gradually increases. Thus, the quantity of heat of unit area/unit time of a flame generated from a torch to the surface of the preform alters, and the quantity of heat at the end of accumulating the optical fiber preform becomes considerably smaller than that at the initial time.
The shrink-fitting degree of the porous glass preform becomes insufficient toward the end of the accumulation due to such a phenomenon so that there is a difference in the density of the optical fiber preform over the radial direction of the preform between the central portion and the peripheral portion.
The density of the porous glass preform is preferably 0.4 to 1.0 g/cm.sup.3. If the density of the porous glass preform decreases below this value due to the insufficient shrink-fitting degree, a crack occurs in the preform along the longitudinal direction of the preform at growing or cooling time.
To eliminate this drawback, the rotating speed of the preform is decelerated in response to the growth of the preform or the quantity of combustion gas is increases.
However, in the former case that the rotating speed of the preform is decelerated, a cause of an uneven surface is produced on the surface of the porous glass preform or an improper outer diameter is produced in the preform.
In the latter case that the quantity of combustion gas is increased, this method depends upon an uncertain process of setting experimentally the increasing amount of the gas and is very difficult to gradually increase the combustible gas to eliminate the uneven accumulating density of the optical fiber preform by preventing the preform from cracking when considering that a flame generated from a torch is of a converging shape.
As described above, the method of fabricating the porous glass preform with a conventional torque of multiwall tube structure can hardly provide a large-size preform having uniform optical fiber preform density without crack nor improper outer diameter.